Acrylic and methacrylic acid and their esters are perhaps some of the most versatile monomers for improving performance characteristics of thousands of polymer and copolymer formulations. Major markets for copolymers of acrylic or methacrylic acid with acrylic or methacrylic esters include the fields of surface coatings, dispersions, textiles, adhesives, paper, and plastics. Acrylic acid/acrylate ester copolymers are considered as non-toxic.
The copolymerization of acrylic or methacrylic acid is generally known. The formation of copolymers from monomers is known to be a non-economic method. This is due to the cost of acrylic and methacrylic esters. On the other hand, the production of these copolymers by usual methods, such as esterification of polyacrylic or polymethacrylic acid with alcohol, is known to be based on an equilibrium reaction.
A major problem in the esterification of polyacrylic or methacrylic acid is removing water from the reaction mixture during its production due to the presence of an azeotropic water-alcohol mixture. Sometimes the boiling points of alcohol and water are very close, and sometimes some crosslinking reaction occurs at high temperature. The extraction of water from the reaction mixture using a traditional technique such as distillation is a non-economical method.
The synthesis of acrylic or methacrylic acid/acrylate or methacrylate ester copolymers directly from their corresponding monomer acids after esterification shows several problems.
A major problem in the esterification of acrylic or methacrylic acid is the high tendency of these monomers to polymerize owing to their reactive double bonds. This is true in particular if the acrylic or methacrylic acid is exposed to relatively high temperatures, cf. for example, PCT Publication No. WO 97/37962. In their preparation, purified ester compounds are exposed to temperatures which can readily trigger polymerization and can lead to polymer formation. This results in soiling the apparatus, pump, coating of column trays, and heat exchanger surfaces (fouling). Cleaning is complicated, expensive, and presents environmental problems, cf. German Patent No. DE 10 67 806 A. In addition, availability is greatly reduced.
As a rule, polymerization inhibitors (phenothiazine or p-methoxy-phenol)1, i.e., compounds which are capable of substantially suppressing radical polymerization, therefore are added for stabilization. For using acrylates or methacrylates, however, the polymerization inhibitors have to be separated off. In the preparation of high-boiling point compounds which cannot be purified by distillation, it is possible to use only inhibitors which can be extracted by another method, for example, by extraction, filtration, or adsorption, or which do not interfere with processing. Thus, German Patent No. DE 28 38 691 A describes the use of Cu(I) oxide as an inhibitor. The Cu(I) can be removed by extraction. PCT Publication No. WO 90/07487 describes the use of hydroquinone addition with the active esterification mixture. The active carbon, which is filtered off after the esterification, is added in order to avoid discoloration of the ester due to the hydroquinone. U.S. Pat. No. 6,726,854 discloses the use of phosphates, such as triethyl phosphate, as a polymerization inhibitor.
Various acrylic esters are useful chemicals.2 Esterification of acrylic acid with alcohol has commercially been performed by using liquid catalysts such as sulfuric acid, hydrofluoric acid, and para-toluenesulfonic acid; however, these are toxic, corrosive, and often hard to remove from the reaction solution.3-5 
The esterification of acrylic or methacrylic acid generally is effected in a reactor to which a distillation condenser is attached. The said column serves to remove water from the reactor in the form of a mixture with the solvent. Reactors mostly used are stirred reactors with a double-wall. Their disadvantage here is that the stirrers frequently need to be repaired, and polymer is readily accumulated. Moreover, the reactor size is subject to limits because of the specific wall area available for heating with increasing reactor size.
Thus, it is an object of the present invention to provide an improved method for the synthesis of acrylic or methacrylic acid/acrylate or methacrylate ester copolymers or terpolymers.